1. Database Management System
Lecture - 37
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Introduction
Any subset of the fields of a relation can be the search key for an index on the relation.
Search key is not the same as key (e.g. doesn’t have to be unique ID).
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Introduction
An index contains a collection of data entries, and supports efficient retrieval of all records with a given search key value k.
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Introduction
Typically, index also contains auxiliary information that directs searches to the desired data entries
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Introduction
Can have multiple (different) indexes per file.
e.g. file sorted by stid, with a hash index on cgpa and a B+tree index on stname.
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Introduction
Indexes on primary key and on attribute(s) in the unique constraint are automatically created
We can still create more
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Index Classification
What selections does it support?
Representation of data entries in index
what kind of info is the index actually storing?
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Index Classification
3 alternatives here
Clustered vs. Un-clustered Indexes
Single Key vs. Composite Indexes
Tree-based, inverted files, pointers
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Creating Index
CREATE [ UNIQUE ]
[ CLUSTERED | NONCLUSTERED ]
INDEX index_name
ON { table | view } ( column [ ASC | DESC ] [ ,...n ] )
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Create Index Example
CREATE UNIQUE INDEX pr_prName
ON program(prName)
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Create Index Example
Can also be created on composite attributes
CREATE UNIQUE INDEX
St_Name ON
Student(stName ASC, stFName DESC)
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Properties of Indexes
Indexes can be defined even when there is no data in the table
Existing values are checked on execution of this command
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Properties of Indexes
Support selections of form
field <operator> constant
Support equality selections
Either “tree” or “hash” indexes help here.
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Properties of Indexes
Support Range selections (operator is one among <, >, <=, >=, BETWEEN)
“Hash” indexes don’t work for these
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Secondary Key Index
Users often need to access data on the basis of non-key or non-unique attribute; secondary key
Like student name, program name, students enrolled in a particular program
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Secondary Key Index
Records are stored on the basis of key attribute; three possibilities
Sequential search
Sorted on the basis of name
Sort for command execution
Problematic
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Secondary Key Index
Index on secondary key is solution
Data remains in original order, however secondary key index maintains ordered sequence of records, and provides direct access
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Secondary Key Index
Three implementation approaches
Inverted files or inversions
Linked lists
B+ Trees
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19. Database Management System
Lecture - 37
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